lseek

Prototype:

off_t lseek(int fd , off_t offset , int whence );

lseek(2) uses the type off_t. This is a 32-bit signed type on 32-bit architectures, unless one compiles with

#define _FILE_OFFSET_BITS 64

in which case it is a 64-bit signed type.

lseek64

Prototype:

off64_t lseek64(int fd , off64_t offset , int whence );

The library routine lseek64() uses a 64-bit type even when off_t is a 32-bit type. Its prototype (and the type off64_t) is available only when one compiles with

#define _LARGEFILE64_SOURCE

The function lseek64() is available since glibc 2.1, and is defined to be an alias for llseek().

llseek

Prototype:

loff_t llseek(int fd , loff_t offset , int whence );

The type loff_t is a 64-bit signed type. The library routine llseek() is available in libc5 and glibc and works without special defines. Its prototype was given in <unistd.h> with libc5, but glibc does not provide a prototype. This is bad, since a prototype is needed. Users should add the above prototype, or something equivalent, to their own source. When users complained about data loss caused by a miscompilation of e2fsck(8), glibc 2.1.3 added the link-time warning
"the `llseek´ function may be dangerous; use `lseek64´ instead."
This makes this function unusable if one desires a warning-free compilation.

_llseek

All the above functions are implemented in terms of this system call. The prototype is:

int _llseek(int fd , off_t offset_hi , off_t offset_lo ,
loff_t * result , int whence );

For more details, see llseek(2).